1. Field of the Invention
The present invention relates to a mobile communication system and more particularly, to a method of controlling power for a forward common channel.
2. Background of the Related Art
Generally, in W-CDMA mobile communication system, data is transmitted using orthogonal codes by synchronizing symbols of channels transmitted from a base station through forward links. As a result, the number of orthogonal codes used for distinguishing a plurality of mobile stations diminishes as the number of mobile stations and/or transmission rate increases. Thus, for a more efficient management of the limited number of orthogonal codes, a forward channel with one orthogonal code, namely a forward common data channel, is commonly assigned to a plurality of mobile stations and data is transferred to respective mobile stations based on time division.
FIG. 1 shows a forward common data channel (FCDCH) in the related art. A FCDCH is a channel that can be shared by a plurality of stations and a base station assigns an identical orthogonal code to respective mobile stations to form a FCDCH. The base stations distinguishes each of the plurality of mobile stations based upon time division and depending upon a transmission time of each such mobile stations, data which is not sensitive to delay is transmitted to respective mobile stations in frame units. FIG. 2 shows structures of a FCDCH in FIG. 1 and a Forward Common Data Control Channel (FCDCCH).
Referring to FIG. 2, a base station transmits a pilot symbol in every slot to estimate a channel state and a Signal Interference Ratio (SIR) of a forward link. The FCDCH also receives through a reverse link a Power Control Bit (PCB) in every slot from each of the mobile stations to perform a fast close loop power control. Accordingly, the FCDCH is transmitted in parallel with the FCDCCH which transmits the control information of the FCDCH and which performs a reverse link power control. Particularly, the FCDCCH broadcasts reverse link PCBs and control information of respective mobile stations to all used mobile stations in a cell. Here, the control information may be an FCDCH assignment message. Moreover, FCDCCH is transmitted at a fixed power without being fast close loop power controlled.
Furthermore, in a W-CDMA communication system, a Dedicated Physical Data Channel (DPDCH) is implemented for transmission of data in I channel signals of a dedicated reverse link channel; and a Dedicated Physical Control Channel (DPCCH) is implemented for transmission of pilot symbols, PCBs and control information of layer 1 in Q channel signals. A mobile station does not transmit the DPDCH if there are no data to be transmitted to a base station. On the other hand, mobile stations continue to transmit the pilot symbols and the PCBs through the DPCCH to maintain reverse link synchronization and forward link power control, even if there are no data to be transmitted to a base station.
However, if a base station in the related art transmits data to the plurality of mobile stations through a forward common channel, the close loop power control may not be maintained between the base station and a mobile station, causing a reduction of a user capacity and deterioration of the system performance. For example, if a base station transmits data to a mobile station through a forward common data channel by time synchronization of three frame, a power control of the mobile station for a first frame is not performed while a second and third frames are transmitted after the transmission of the first frame. Therefore, if data is to be transmitted to the mobile station in a fourth frame, a power control cannot be performed since the base station does not know an initial power value for restarting a transmission to the mobile station.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide an efficient method for controlling power of a forward common channel.
Another object of the present invention is to provide a method for controlling power of a forward common channel in which an initial power is obtained for use in transmission of data to a mobile station.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, a method for controlling power of a forward common channel set between a plurality of mobile stations and a base station in a communication system includes (1) measuring from a mobile station a first reception power of a FCDCCH at an end of data reception from the base station, (2) measuring from the base station a transmission power of a FCDCH at an end of data transmission to the mobile station, (3) receiving at the mobile station a channel assignment message from the base station, (4) measuring from the mobile station a second reception power of the FCDCCH when the channel assignment message is received, (5) calculating at the mobile station a difference of the first and second reception powers, and forwarding the difference to the base station, and (6) determining at the base station an initial power of the data transmitted to the mobile station using the transmitted difference and the measured transmission power of the FCDCH.
In another embodiment of the present invention, a method for controlling power of a forward common channel set between a plurality of mobile stations and a base station in a communication system includes (1) measuring from a mobile station a first reception power of an FCDCCH at an end of data reception from the base station, and setting the first reception power as a power control critical value, (2) measuring from the base station a transmission power of an FCDCH at an end of data transmission to the mobile station, (3) measuring from the mobile station second reception powers of the FCDCCH at fixed intervals, and transmitting PCBs to the base station according to a difference between the measured second reception power and the power control critical value, (4) accumulating at the base station and storing the PCBs transmitted from the mobile station, (5) receiving a channel assignment message at the mobile station, and (6) determining at the base station an initial power of the data to be transmitted to the mobile station using the accumulated PCBs and the transmission power of the FCDCH, upon reception of the channel assignment message at the mobile station.